On Wed, 21 Jan 2004 16:16:10 +0000, Dean Richard Benson
wrote:

I have just had my loft timbers/joists treated for woodworm, and getting
the house rewired soon. After that is all done, I am going to put
insulation down. However I have some questions about it.

Is there a noticable difference in loft insulation (rockwool rolls) in
insulating greater than a 4" depth? The reason I ask is that my joists
are 4" in height, and anything greater will mean a bit more planning to
ensure that I can still walk and see the joists up there.

Also, has anyone noticed a difference between rockwool and other types,
such as crown wool? I have checked the u-values and such, and rockwool is
a better insulator, but it is also going to cost me about 20% more than
crown insulation. (b&q special offer).

I really like the idea of using natural insulation such as sheep wool :
(http://www.greenbuildingstore.co.uk/ins-thermperf.php) but the price is
unbelievable.

Any comments/help/advice would be greatly received.

Thanks.

You can do the sums on this.

Calculate the heat loss for the area of the ceilings.

For a pitched roof plus 100mm of insulation, the U value is 0.35

So, let's say the house is 7m square for the sake of argument.

Using conventional central heating temperature assumptions of -3
degrees worst case outside and 18 degrees in the upstairs rooms, the
heat loss will be

Area x U x temperature difference

or 7 x 7 x 21 x 0.35 = 360 Watts

If you have 150mm of insulation, the U value falls to 0.25 and the
heat loss will be 257W.

With no insulation, the U value of the pitched roof is about 2.0 and
the heat loss about 2kW.

You can see that there is a huge difference between nothing and 100mm,
but a rapidly diminishing return after that.

This is typical worst case.

However, averaged over the year, the outside temperature is probably
closer to 10 degrees, so assuming continuous heating at 100mm of
insulation the loss is 223Watts and at 150mm about 160W. With 200mm
you would get down towards 120W.

100W of heating by gas at a price of 1.4p per kWh works out to just
over £12 per annum.

In terms of the material cost, this provides a reasonable rate of
return. However, if you set it in the context of whether the space
is important to you and the cost of the timber and other means to
access it, then this becomes less interesting.

In energy terms, if you compare with the heat loss through the walls
and other means, then you realise that the focus should be on other
issues than this.







..andy

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